1. Field of the Invention
The present invention relates to a light-emitting device including a light-emitting element that emits light by applying a current, and in particular, relates to a light-emitting device including a light-emitting element with smaller luminance degradation, and a driving method of a light-emitting element for reducing luminance degradation.
2. Description of the Related Art
In recent years, a light-emitting element from which a high luminance can be obtained by applying a current, such as a light-emitting diode (LED) and a light-emitting element using a luminescent organic compound (OLED: organic light-emitting diode), has been getting a lot of attention.
The fundamental structure of a light-emitting element using a luminescent organic compound has a layer including a luminescent organic compound between a pair of electrodes. By applying a voltage to this element, an electron and a hole from a pair of electrodes are transported to the layer including the luminescent organic compound so that a current flows. Then, the luminescent organic compound forms an excited state by recombining those carriers (the electron and the hole), and luminescence is produced when the excited state returns to the ground state.
It is to be noted that a singlet excited state and a triplet excited state are possible as the type of an excited state formed by an organic compound and that luminescence from the singlet excited state and luminescence from the triplet excited state are respectively referred to as fluorescence and phosphorescence.
This light-emitting element, which is usually formed to be a submicron to several-micron thin film, has the great advantage of being able to be manufactured to be thin and light. In addition, the time from carrier injection till light emission is approximately a microsecond or less, and the quite high response speed is one of features. Further, the power consumption is relatively small since enough luminescence can be obtained at a direct-current voltage on the order of several volts to several tens of volts. From the viewpoint of these advantages, the light-emitting element described above has been attracting attention as a next-generation flat-panel display element.
In addition, in this light-emitting element, the pair of electrode and the light-emitting layer including the luminescent organic compound are formed in the shape of a film. Therefore, planar light emission can be easily obtained by forming a large-area element. Since this is a feature that is hard to obtain from a point source typified by an incandescent bulb or an LED or from a line source typified by a fluorescent light, the light-emitting element also serve many uses as a plane source that can be applied to lighting and the like.
Meanwhile, the output (luminance) of a light-emitting element that emits light by applying a current as described above is determined by the applied amount of current. Therefore, when the light-emitting element is made to emit light, a suitable luminance can be achieved as usage by controlling the amount of current to an appropriate value. It is to be noted that the ratio of the luminance to the current density is referred to as a current efficiency in this case.
As long as this current efficiency does not change, a constant luminance is supposed to be obtained by applying a constant current. However, the reality is that the current efficiency changes. Since the current efficiency of a normal light-emitting element gradually decreases by applying a current (or emitting light), the luminance thereof gradually decreases even when a constant current is applied. In particular, in a light-emitting element using a luminescent organic compound, this luminance degradation appears prominently, which acts as a drag on development of light-emitting elements.
Therefore, in research and development in this field, a lot of improvements in material and device structure have been made in order to suppress luminance degradation as much as possible when a constant current is kept flowing. In the result, nowadays, light-emitting elements that achieve several tens of thousands of hours in luminance half-life have been further developed, and many people say the light-emitting elements can be put into practical use. It is to be noted that constant current drive indicates that a current is kept flowing at a constant current density.
However, since difference in luminance (or unevenness of luminance) is recognized even to a several % extent, sufficient reliability has not been achieved yet for application fields that require smallness of luminance degradation, for example, display uses such as personal computers and televisions or lighting uses. Although a high luminance is required particularly for lighting, the level for practical use has not been reached yet in view of the present situation in light-emitting element that the higher a preset luminance is, the fast luminance degradation proceed.
The mechanism of this luminance degradation has not been sufficiently discussed yet. However, for example, there is a reference in which a luminance degradation curve (time-luminance curve) for the case of keeping a constant current flowing is subjected to fitting by a function referred to as an stretched exponential function, and the mechanism of luminance degradation is considered (refer to Non-Patent Reference 1). Luminance degradation has a plurality of causes intricately intertwined with each other, and unfortunately, has not been elucidated fundamentally. However, this function itself is capable of quite precise fitting for a luminance degradation curve.
At any rate, the luminance degradation described above, that is, the decrease in current efficiency, is basically believed to be caused mainly by low decay durability of an organic material and fragility of an organic thin film, it can be said that improvements in material and device structure are not enough. Consequently, an attempt to suppress luminance degradation from a viewpoint of a driving method has been made (for example, refer to Patent Reference 1). In Patent Reference 1, occurrence itself of luminance degradation has not been suppressed significantly although the luminance half-life is improved to be twice as long by applying a reverse bias.    (Non-Patent Reference 1) Masahiko Ishii, et al., Applied Physics Letters, vol. 80(18), 3430-3432 (2002)    (Patent Reference 1) Japanese Patent Laid-Open No. 2003-323988